Continental export of si to the coastal zone is closely linked to the ocean carbon sink and to the dynamics of phytoplankton blooms in coastal ecosystems. Presently, however, the impact of human cultivation of the landscape on terrestrial si fluxes remains unquantified and is not incorporated in models for terrestrial si mobilization. In this paper, we show that land use is the most important controlling factor of si mobilization in temperate European watersheds, with sustained cultivation ( > 250 years) of formerly forested areas leading to a twofold to threefold decrease in baseflow delivery of si. This is a breakthrough in our understanding of the biogeochemical si cycle: it shows that human cultivation of the landscape should be recognized as an important controlling factor of terrestrial si fluxes.

We investigated long-term trends in dissolved inorganic nutrients in the tidal part of the Scheldt estuary (Belgium, The Netherlands). Annually averaged concentrations of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), and phosphate (DIP) increased significantly until the mid-1970s, after which they declined linearly at rates of 0.6, 2.9, and 0.3 mol L Ϫ1 yr Ϫ1 , respectively. This co-occurred with a deterioration followed by a restoration of water column oxic conditions. Because of the differences in the reduction rate of DSi (1.2% yr Ϫ1 ), DIN (1.7% yr Ϫ1 ), and DIP (5.4% yr Ϫ1 ), the N : P and Si : P ratios more than doubled from 1980 to 2002. The Si : N ratio varied from 0.2 to 0.4 and was positively correlated with river discharge. The part downstream from the confluence of the main rivers was a net sink for DSi during the entire period but evolved from a net sink to a net source for DIP, while the reverse was true for DIN. This differential behavior of the estuary with respect to DIN and DIP strongly buffered the altered loadings to the upper estuary. The input of oxygen-consuming substances at the head of the estuary triggered a sequence of oxidation reactions. In the early 1970s, high loadings of ammonium and organic matter caused oxygen depletion and intense water-column denitrification in the upstream part and intense nitrification downstream, with a nitrate maximum succeeding a nitrite peak. With oxic conditions improving and the input of ammonium decreasing, water-column denitrification declined, the nitrification front migrated upstream, and the estuary evolved from a net producer of nitrite to a net consumer. Now, at the beginning of the 21st century, nitrate behaves almost conservatively over the entire estuary.

Estuaries are naturally highly dynamic and rapidly changing systems, forming a complex mixture of many different habitat types. They are very productive biomes and support many important ecosystem functions: biogeochemical cycling and movement of nutrients, mitigation of floods, maintenance of biodiversity and biological production. Human pressure on estuaries is very high. On the other hand, it is recognized that estuaries have a unique functional and structural biodiversity. Therefore, these ecosystems are particularly important for integrating sound ecological management with sustainable economics. These opportunities are explored for the Scheldt estuary, a well-documented system with an exceptional tidal freshwater area. In this article a description of the Scheldt estuary is presented, illustrating that human influence is intertwined with natural dynamics. Hydrology, geomorphology, trophic status and diversity are discussed, and possible future trends in both natural evolution and management are argued.

Silicon is one of the most important elements in the current age of the anthropocene. It has numerous industrial applications, and supports a high-tech multibillion Euro industry. Silicon has a fascinating biological and geological cycle, interacting with other globally important biogeochemical cycles. In this review, we bring together both biological and geological aspects of the silicon cycle to provide a general, comprehensive review of the cycling of silicon in the environment. We hope this review will provide inspiration for researchers to study this fascinating element, as well as providing a background environmental context to those interested in silicon.The Earth's crust consists primarily of silicates (Si oxides, 90% of all minerals); consequently silicon (hereafter referred to as Si) is the second-most abundant element in the earth's crust (28.8%) after oxygen [1]. During silicate weathering dissolved soil CO 2 is used in a reaction where ortho-silicic acid (H 4 SiO 4 ) is dissolved and released from the crystalline structure of silicate minerals. In the environment dissolved silicate (DSi), i.e. ortho-silicic acid (H 4 SiO 4 ), is transported through soil and exported to rivers and eventually the ocean [2] (Fig. 1). The silicate weathering process consumes CO 2 . For example, in the weathering of anorthite (over kaolinite) to gibbsite, DSi is produced and CO 2 is consumed [3]: CaAl 2 Si 2 O 8 þ2:CO 2 þ8:H 2 O!Ca 2þ þ2:Al OH ð Þ 3 þ2:H 4 SiO 4 þ2:HCO À 3Ca 2þ þ 2HCO À 3 ! CaCO 3 þ H 2 CO 3

We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole-ecosystem 15 N labeling experiment. 15 N-NH was added to the floodwater entering a 3,477 ϩ 4 m 2 tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO , NO , N 2 O, N 2 , NH , and suspended particulate nitrogen (SPN) were measured in concert with a mass balancestudy. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br Ϫ budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added for 30% of 15 N-transformation. In situ whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation. Under conditions of low oxygen concentrations and high ammonium availability, nitrifiers produced N 2 O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.

We studied the seasonal exchange of biogenic silica (BSi) and dissolved silica (DSi) between a freshwater and a saltwater tidal marsh and the adjacent coastal waters. Export of DSi was observed from both tidal marshes, whereas BSi was imported in association with suspended solids. The export of DSi was highest (23.4% and 123.8% in the freshwater and saltwater marsh, respectively) in summer when DSi concentrations were low in the nearby coastal waters. Combined data from both marshes suggested a logarithmic decrease in DSi export with increasing DSi concentrations in the inundating waters. BSi import was observed year round in the freshwater marsh, but only in summer in the saltwater marsh. The results show that DSi export from tidal marshes, both freshwater and salt water, contributes significantly to estuarine Si availability in summer and provide new insights regarding potential linkages between tidal marshes and secondary production in nearby coastal waters.Compared with our extensive knowledge concerning N and P processing in the aquatic continuum of watersheds, rivers, lakes, and estuaries, the transport and cycling of silicon has been significantly less well studied (Conley et al.

This paper presents the results of 7 years of integrated monitoring along the Scheldt estuary. The combination of two datasets resulted in a full description of the estuaries water quality parameters from the mouth to the upper boundary, including an extended fresh water tidal part. A synthesis of the monitoring results and all relevant ecological knowledge on the Scheldt allowed to identify opportunities to optimize its management. The results show that the effect of discharge on salinity has a distinct maximum in the polyhaline to mesohaline transition area. Oxygen conditions, nitrogen removal and phytoplankton regulation can be enhanced and improved through management measures within the estuary. To lower carbon and phosphorous loads however measures should be taken within the catchment. To restore most of its ecological functions the estuary needs more space. Optimal locations to address specific functions can be derived from the monitoring results.

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